Per-pixel performance improvement for combined visible and infrared image sensor arrays

ABSTRACT

A per-pixel performance improvement is described for combined image sensor arrays that measure infrared and visible light. One embodiment is a method that includes forming an array of photodetectors on a silicon substrate, treating a subset of the photodetectors to improve sensitivity to infrared light, and finishing the photodetector array to form an image sensor.

FIELD

The present description relates to the field of image sensors for nearinfrared and, in particular, to an image sensor with pixels that areconfigured for particular optical bands.

BACKGROUND

Small image sensors continue to improve in cost and performance and havebecome ubiquitous in smart phones, notebook computers, tablets and manyother devices. At the same new device types such as headsets, glasses,dashboard cameras, and autonomous vehicles continue to emerge. Thecommon CMOS (Complementary Metal Oxide Semiconductor) image sensor thatis used in most digital cameras has an array of photodetectors. Usuallythere is one photodetector for each pixel. The sensor is well suited tocapture and measure visible light.

The same sensor is also able to capture and measure NIR (Near Infrared)light. As a result, new applications are being developed to exploit thisproperty. Biometric authentication and depth cameras, for example havebeen developed to use NIR. NIR has a benefit of revealing features thatare not visible in visible light. Such features may reflect NIR but notvisible light or the system may incorporate invisible, NIR illuminationthat does not distract or otherwise influence the user.

Imaging applications extending the range of spectral sensitivity beyondthe visible range are coming to market. These applications include facerecognition, iris scanning, and multi-spectral imaging for analyzingchemical content to name a few examples.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The material described herein is illustrated by way of example and notby way of limitation in the accompanying figures. For simplicity andclarity of illustration, elements illustrated in the figures are notnecessarily drawn to scale. For example, the dimensions of some elementsmay be exaggerated relative to other elements for clarity.

FIG. 1 is a graph of photon absorption of various materials.

FIG. 2 is a cross-sectional side view diagram of a portion of an imagesensor array with selective pixel enhancement according to anembodiment.

FIG. 3 is a cross-sectional side view diagram of a portion of an imagesensor array with uniform pixels according to an embodiment.

FIG. 4 is a cross-sectional side view diagram of a portion of the imagesensor array of FIG. 3 with a hard mask opened for selective pixelenhancement according to an embodiment.

FIG. 5 is a cross-sectional side view diagram of a portion of the imagesensor array of FIG. 4 with the hard mask removed according to anembodiment.

FIG. 6 is a cross-sectional side view diagram of a portion of the imagesensor array of FIG. 5 with selective pixel enhancement after finishingaccording to an embodiment.

FIG. 7 is a block diagram of an image sensor with multiplephotodetectors and depth sensing according to an embodiment.

FIG. 8 is a block diagram of a computing device incorporating depthsensing and high dynamic range according to an embodiment.

DETAILED DESCRIPTION

The proposed solution optimizes the bandgap of the silicon material on aper pixel basis, enabling optimization of the Quantum Efficiency (QE)and detector leakage based on the wavelength being detected at eachpixel. This allows visible and infrared light performance to be enhancedon a single sensor or an image array. Such sensors are useful not onlyfor biometric identification but also for lighting control and materialsscanning, such as checking food.

CMOS image sensors provide good performance for visible light and alsofor infrared light. They have a common underlying photodetector elementthat is optimized for the visible spectrum due to the large focus in themarketplace on visible light photography. This leads to a lowerperformance in the near infrared (NIR) wavelengths which are consideredless important in the marketplace. As described herein, each pixel maybe configured to provide high performance for its intended purposewithout affecting the other pixels.

The most common material used to build photodetectors for imagingapplications is silicon. The spectral absorption of silicon at longerwavelengths is reduced due to the band-gap of silicon which is about1.14 eV. As photons approach this energy level, the probability ofabsorption for a given penetration depth decreases and for photons belowthis energy level, silicon is transparent. Photons travel right throughthe silicon and energy cannot be absorbed and detected.

Other materials are used for optical applications and may be used forimage sensors such as germanium and gallium arsenide. FIG. 1 is a graphof the absorption coefficient on the vertical axis for different lightwavelengths on the horizontal axis for three materials. The curve 102for silicon shows a very steep drop in absorption as the wavelengthincreases. At about a 1.1 μm wavelength there is almost no absorption. Acurve 104 for GaAs shows a much higher absorption coefficient up toabout 0.85 μm in the wavelength of incident light. For longerwavelengths there is almost no absorption. A curve 106 for germanium hashigher absorption at wavelengths longer than about 0.4 μm and up to 1.5μm after which the absorption drops almost immediately. Germanium isnaturally a better material for an infrared photodetector.

While these other materials provide higher sensitivity to infrared lightthan silicon they are also more expensive, more difficult to turn intophotodetectors, and less suited to fabricating logic circuits. Inaddition, decreasing the band gap by using a more responsive materialalso increases the probability of thermal energy causing an electron tojump across the gap. This may cause higher leakage between pixels, andtherefore higher noise. The noise, in this case shows as randomvariations in the dark level of different photosites as the number ofthermally generated electrons varies with time, process, andtemperature.

As described herein, the band gap of selected photodetectors in an arraymay be modified without affecting the other photodetectors. This changesthe absorption characteristics of the silicon on a per-pixel basis.Individual photodetectors may be modified using standard siliconprocessing techniques.

FIG. 2 is a cross-sectional side view diagram of a portion of a row ofphotodetectors of an image sensor array placed side-by-side. The pixelfor each photodetector is part of a back-side illuminated array,however, embodiments are not so limited. The pixels are formed over asubstrate 122 that includes wiring layers 124 for power, exposurecontrol, and any other desired functions. Each pixel is formed in ann-well of a p-substrate 125 that is applied over the back side of thewiring layer silicon substrate 122. The n-well is the active photodiodethat converts photons to electrons. A typically p+ surface passivationlayer 127 is applied over the photodiodes and a color filter and afocusing lens with an anti-reflective (AR) coating is applied over thepassivation layer.

Incident light strikes the AR coating and lens and penetrates throughthe surface passivation 127 and the p-substrate 125 to the n-well 128.The photons of the incident light are then converted to free electronswhich can be measured in the circuitry of the wiring layers on the frontside of the substrate 122. The image sensor is made by flipping over thesubstrate after the circuitry is formed on the front side of thesubstrate. The photodetectors 129 are built on the backside of thesilicon substrate 122. An optical barrier layer 126 is formed over theback side of the substrate and then the photodetectors 128 are formedover the barrier layer. A color filter is formed over each photodetectorand additional optics, e.g. a condensing, collimating, or focusing lens129 is formed over the color filter.

In this example, the row has a repeating pattern of red 130, green 132,blue 134, and infrared 136 color filters. This pattern repeats on eachrow. Image sensor arrays typically have some other more complex patterninstead of the even distribution of RGB, and IR as shown. One suchpattern is a Bayer pattern with twice as many green color filters as redor blue. This pattern exploits the increased sensitivity to green inhuman perception. The resulting image often seems more detailed to ahuman eye than an image from an RGB array with the same number ofpixels.

The color filters limit the light that is transmitted through the filterto the respective color. For the red filter, red light passes throughthe filter. Other light, such as green, blue, and infrared is reflectedor absorbed. For red, green, and blue light, the standard absorptioncharacteristics of silicon will serve well as photodetectors. Thesephotodetectors 128 are made in a conventional way and have a standardsilicon bandgap.

The infrared photodetector 138, that is the photodetector under theinfrared color filter, may be modified so that it responds better toinfrared light. If this modification reduces its response to visiblelight, the performance of the array will not be reduced because theinfrared filter will block any visible light from ever impinging on thephotodetector area.

As shown in FIG. 2, red, green, blue and IR pass filters, 130, 132, 134,136 are laid on top of silicon based photodiodes 128 as in traditionalmulti-spectral imagers. The difference in this case is that thephotodiodes 138 under the NIR pass filters are processed to reduce theeffective band gap of the silicon. This increases the absorption of thephotodetector for NIR and also increases the leakage from thermallygenerated electrons. The total leakage may be reduced by only treatingthe NIR photodetectors and not the other three types. Such an increasein leakage would be detrimental to the accuracy of the RGB pixels, sothe RGB pixels are not processed and have the standard silicon band gapand leakage characteristics. In the examples herein, the treating may bein the form of doping the IR photodetectors with chalcogens, in the formof irradiating the IR photodetectors, or in any other desired form.

For the doping approach, the original silicon bad gap energy is 1.14 eV.This can be reduced to about 1.12 eV using doping. This reduced bandgapcan be obtained by hyper doping the silicon n-well with chalcogens, e.g.sulfur, selenium, or tellurium, beyond the equilibrium solubility limit.The hyper doped silicon exhibits sub-band gap light absorption, makingit a suitable material for silicon based infrared (IR) detection. Thedoping can be controlled so that only the IR pixel 138 receives thechalcogen doping using photolithography or other silicon processingtechnologies.

For irradiation, laser irradiation may be applied selectively to onlythe NIR subpixel surfaces. This is done either in the presence ofappropriate background gases that contain chalcogens or after depositinga chalcogen layer as a powder or film over the surface of the silicon.When the silicon is irradiated it forms an energy band of impuritystates that overlap with the silicon conduction band edge. This band ofimpurities reduces the band gap from 1.1 eV to approximately 0.4 eV.

Chalcogens have a low solid solubility limit in silicon. This may beovercome by applying femtosecond or nanosecond laser annealing tochalcogen implanted silicon or to bare silicon in certain backgroundgases. Alternatively, rapid thermal annealing may be used. The laserenergy density causes a liquid phase on the silicon surface during thepulse and then a fast recrystallization velocity in between pulses. Thiscauses some of the chalcogens to be trapped in the silicon matrix.However, this method encounters the problem of surface segregation.

These techniques are able to infuse the chalcogen impurities into thesilicon. The impurities transform the silicon photodetector to have anear-unity broadband absorption below the Si bandgap, a highergeneration of photocurrent below the bandgap, and an insulator-to-metaltransition.

FIGS. 3-6 are cross-sectional side view diagrams of a portion of animage array to show a process for forming a photodetector array such asthat shown in FIG. 2. FIG. 3 has a silicon substrate 140 withphotodetector n-wells 144 formed in the substrate. The n-wells areformed over a base substrate 146 which may have wiring layers and otherelectrical components formed therein.

A hard mask 142 of e.g. Si₃N₄ or SiO₂ is formed or deposited over eachof the photodetectors. Instead of a hard mask any other protectivecovering or material may be used that has a low diffusivity of Sulfur.The mask 142 may be deposited or applied in any of a variety ofdifferent ways including PECVD (Plasma Enhanced Chemical VaporDeposition) or ALD (Atomic Layer Deposition). A safe temperature forsilicon may be used, such as <500° C.

In this example, the hard mask is directly on top of the pixels. Fourpixels are shown to indicate red, green, blue and NIR subpixels. Thepattern and arrangement may be different from that shown and a typicalarray will have millions of sub-pixels, although the invention is not solimited.

FIG. 4 is a similar cross-sectional side view of the same portion of thesubstrate 140 and hard mask 142. At this stage, the mask is etched awayover the NIR pixels 150 to form an opening 156. The mask may be etchedin any of a variety of different ways, for example, a sub micrometerscale lithography technology or method may be used to selectively etchaway the mask layer from over the NIR subpixel area. As shown thevisible light pixels 144 are not affected. The mask is only removed overthe NIR pixels.

With the NIR pixels exposed, impurities 158 are driven into only the NIRpixels 150 in order to increase the sensitivity of those pixels to NIRlight. The physical and electrical characteristics of the NIR pixels 150are changed in comparison to those of the visible light pixels 144. Thismay be done in a variety of different ways as mentioned above. Theseways include hyperdoping, laser irradiation and other techniques.

In one example, a UV (Ultraviolet) light laser, e.g. at 200 nm-300 nmwavelength, is used to irradiate the entire structure with the selectivehard mask openings 156. The UV laser is applied in an ambient of SF₆ orH₂S gases to selectively incorporate a large amount (e.g. 0.3-1 atomicpercentage) of sulfur into the silicon only in the NIR subpixel area.Tellurium or selenium may be incorporated using H₂Te or H₂Se gases,respectively. Other chalcogens and other impurities may alternatively beused.

In another example a layer of powder or film containing chalcogens isapplied over the hard mask so that the film is over the silicon of theNIR pixels but not over the silicon of the visible light pixels. Thesepixels are protected by the hard mask. The same UV laser treatment isapplied with the film instead of with the gases to incorporate thechalcogens in the film into the silicon of the NIR pixels.

FIG. 5 is a similar cross-sectional side view of the same portion of theimage sensor array after the mask has been removed. The mask may beremoved using an appropriate wet or dry etching method, depending on thenature of the mask. As a result the impurities are only in the NIR pixeland the other pixels are not changed through this selective process oftreating only the NIR pixels.

FIG. 6 is a similar cross-sectional side view of the same array. Theprocessing is continued to finish the image sensors including depositingcolor filters 152, forming micro lenses 154, applying coatings,barriers, diffusions, etc.

FIG. 7 is a block diagram of an image sensor or camera system 700 thatmay include pixel circuits with infrared light enhancement properties asdescribed herein. The camera 700 includes an image sensor 702 withpixels typically arranged in rows and columns. Each pixel may have amicro-lens and tuned bandgap photodetectors as described above. Eachpixel is coupled to a row line 706 and a column line 708. These areapplied to the image processor 704.

The image processor has a row selector 710 and a column selector 712.The voltage on the column line is fed to an ADC (Analog to DigitalConverter) 714 which may include sample and hold circuits and othertypes of buffers. Alternatively, multiple ADC's may be connected tocolumn lines in any ratio optimizing ADC speed and die area. The ADCvalues are fed to a buffer 716, which holds the values for each exposureto apply to a correction processor 718. This processor may compensatefor any artifacts or design constraints of the image sensor or any otheraspect of the system. The complete image is then compiled and renderedand may be sent to an interface 720 for transfer to external components.

The image processor 704 may be regulated by a controller 722 and containmany other sensors and components. It may perform many more operationsthan those mentioned or another processor may be coupled to the cameraor to multiple cameras for additional processing. The controller mayalso be coupled to a lens system 724. The lens system serves to focus ascene onto the sensor and the controller may adjust focus distance,focal length, aperture and any other settings of the lens system,depending on the particular implementation. For stereo depth imaging, asecond lens 724 and image sensor 702 may be used. This may be coupled tothe same image processor 704 or to its own second image processordepending on the particular implementation.

The controller may also be coupled to a lamp or projector 724. This maybe an LED in the visible or infrared range, a Xenon flash, or anotherillumination source, depending on the particular application for whichthe lamp is being used. The controller coordinates the lamp with theexposure times to achieve different exposure levels described above andfor other purposes. The lamp may produce a structured, coded, or plainillumination field. There may be multiple lamps to produce differentilluminations in different fields of view.

FIG. 8 is a block diagram of a computing device 100 in accordance withone implementation. The computing device 100 houses a system board 2.The board 2 may include a number of components, including but notlimited to a processor 4 and at least one communication package 6. Thecommunication package is coupled to one or more antennas 16. Theprocessor 4 is physically and electrically coupled to the board 2.

Depending on its applications, computing device 100 may include othercomponents that may or may not be physically and electrically coupled tothe board 2. These other components include, but are not limited to,volatile memory (e.g., DRAM) 8, non-volatile memory (e.g., ROM) 9, flashmemory (not shown), a graphics processor 12, a digital signal processor(not shown), a crypto processor (not shown), a chipset 14, an antenna16, a display 18 such as a touchscreen display, a touchscreen controller20, a battery 22, an audio codec (not shown), a video codec (not shown),a power amplifier 24, a global positioning system (GPS) device 26, acompass 28, an accelerometer (not shown), a gyroscope (not shown), aspeaker 30, a camera 32, a lamp 33, a microphone array 34, and a massstorage device (such as a hard disk drive) 10, compact disk (CD) (notshown), digital versatile disk (DVD) (not shown), and so forth). Thesecomponents may be connected to the system board 2, mounted to the systemboard, or combined with any of the other components.

The communication package 6 enables wireless and/or wired communicationsfor the transfer of data to and from the computing device 100. The term“wireless” and its derivatives may be used to describe circuits,devices, systems, methods, techniques, communications channels, etc.,that may communicate data through the use of modulated electromagneticradiation through a non-solid medium. The term does not imply that theassociated devices do not contain any wires, although in someembodiments they might not. The communication package 6 may implementany of a number of wireless or wired standards or protocols, includingbut not limited to Wi-Fi (IEEE 802.11 family), WiMAX (IEEE 802.16family), IEEE 802.20, long term evolution (LTE), Ev-DO, HSPA+, HSDPA+,HSUPA+, EDGE, GSM, GPRS, CDMA, TDMA, DECT, Bluetooth, Ethernetderivatives thereof, as well as any other wireless and wired protocolsthat are designated as 3G, 4G, 5G, and beyond. The computing device 100may include a plurality of communication packages 6. For instance, afirst communication package 6 may be dedicated to shorter range wirelesscommunications such as Wi-Fi and Bluetooth and a second communicationpackage 6 may be dedicated to longer range wireless communications suchas GPS, EDGE, GPRS, CDMA, WiMAX, LTE, Ev-DO, and others.

The cameras 32 contain image sensors with pixels or photodetectors asdescribed herein. The image sensors may use the resources of an imageprocessing chip 3 to read values and also to perform exposure control,depth map determination, format conversion, coding and decoding, noisereduction and 3D mapping, etc. The processor 4 is coupled to the imageprocessing chip to drive the processes, set parameters, etc.

In various implementations, the computing device 100 may be eyewear, alaptop, a netbook, a notebook, an ultrabook, a smartphone, a tablet, apersonal digital assistant (PDA), an ultra mobile PC, a mobile phone, adesktop computer, a server, a set-top box, an entertainment controlunit, a digital camera, a portable music player, a digital videorecorder, wearables or drones. The computing device may be fixed,portable, or wearable. In further implementations, the computing device100 may be any other electronic device that processes data.

Embodiments may be implemented as a part of one or more memory chips,controllers, CPUs (Central Processing Unit), microchips or integratedcircuits interconnected using a motherboard, an application specificintegrated circuit (ASIC), and/or a field programmable gate array(FPGA).

References to “one embodiment”, “an embodiment”, “example embodiment”,“various embodiments”, etc., indicate that the embodiment(s) sodescribed may include particular features, structures, orcharacteristics, but not every embodiment necessarily includes theparticular features, structures, or characteristics. Further, someembodiments may have some, all, or none of the features described forother embodiments.

In the following description and claims, the term “coupled” along withits derivatives, may be used. “Coupled” is used to indicate that two ormore elements co-operate or interact with each other, but they may ormay not have intervening physical or electrical components between them.

As used in the claims, unless otherwise specified, the use of theordinal adjectives “first”, “second”, “third”, etc., to describe acommon element, merely indicate that different instances of likeelements are being referred to, and are not intended to imply that theelements so described must be in a given sequence, either temporally,spatially, in ranking, or in any other manner.

The drawings and the forgoing description give examples of embodiments.Those skilled in the art will appreciate that one or more of thedescribed elements may well be combined into a single functionalelement. Alternatively, certain elements may be split into multiplefunctional elements. Elements from one embodiment may be added toanother embodiment. For example, orders of processes described hereinmay be changed and are not limited to the manner described herein.Moreover, the actions of any flow diagram need not be implemented in theorder shown; nor do all of the acts necessarily need to be performed.Also, those acts that are not dependent on other acts may be performedin parallel with the other acts. The scope of embodiments is by no meanslimited by these specific examples. Numerous variations, whetherexplicitly given in the specification or not, such as differences instructure, dimension, and use of material, are possible. The scope ofembodiments is at least as broad as given by the following claims.

The following examples pertain to further embodiments. The variousfeatures of the different embodiments may be variously combined withsome features included and others excluded to suit a variety ofdifferent applications. Some embodiments pertain to a method of formingan image sensor that includes forming an array of photodetectors on asilicon substrate, treating a subset of the photodetectors to improvesensitivity to infrared light, and finishing the photodetector array toform an image sensor.

In further embodiments forming an array comprises forming an array onthe back side of the substrate after forming circuitry on the front sideof the substrate.

In further embodiments treating comprises doping the subset of thephotodetectors with at least one of sulfur, selenium, and tellurium.

In further embodiments doping comprises forming a mask over thephotodetectors that are not of the subset, applying a chemical vapordeposition of chalcogens over the photodetectors that are not masked andapplying a thermal anneal to the photodetectors that are not masked.

In further embodiments treating comprises irradiating the subset of thephotodetectors.

In further embodiments irradiating comprises irradiating in the presenceof chalcogens to drive chalcogen impurities into the subset ofphotodetectors.

In further embodiments treating comprises causing a silicon surface ofthe subset of photodetectors to transition to liquid in the presence ofchalcogens and then to rapidly solidify with some chalcogenincorporated.

In further embodiments applying visible light color filters comprisesapplying a red, a green, or a blue color filter over each photodetectorthat is not treated in a repeating pattern.

Further embodiments include applying infrared light color filters overthe treated photodetectors, and applying visible light color filtersover the photodetectors that are not treated

Some embodiments pertain to an image sensor that includes a siliconsubstrate, a first set of photodetectors on the silicon substrate thathave impurities that improve sensitivity to infrared light, a second setof photodetector on the silicon substrate, that do not have theimpurities, and an optical system to direct visible and infrared lightto the first and the second sets of photodetectors.

In further embodiments the impurities comprise at least one of sulfur,selenium, and tellurium.

In further embodiments the impurities are trapped in the silicon matrixof photodetectors of the first array of photodetectors.

In further embodiments the array is formed on a back side of the siliconsubstrate.

Further embodiments include circuit layers on a front side of thesilicon substrate opposite the circuit layers.

Further embodiments include a plurality of infrared light color filtersone over each photodetector of the first set of photodetectors and aplurality of visible light color filters one over each photodetector ofthe second set of photodetectors.

In further embodiments the visible light color filters are in arepeating pattern over the array.

Some embodiments pertain to a portable computing system that includes aprocessor, a communications chip coupled to the processor to send andreceive images, and an image sensor coupled to the processor having asilicon substrate, a first set of photodetectors on the siliconsubstrate that have impurities that improve sensitivity to infraredlight a second set of photodetector on the silicon substrate, that donot have the impurities, and an optical system to direct visible andinfrared light to the first and the second sets of photodetectors.

In further embodiments the first set of photodetectors is doped with theimpurities.

In further embodiments the first set of photodetectors has a reducedeffective silicon band gap as compared to the second set ofphotodetectors.

In further embodiments the first set of photodetectors is distributedacross the sensor in a repeating pattern with the second set ofphotodetectors.

What is claimed is:
 1. A method comprising: forming an array ofphotodetectors on a silicon substrate; treating a subset of thephotodetectors to improve sensitivity of material of the photodetectorto infrared light without improving sensitivity to visible light; andfinishing the photodetector array to form an image sensor.
 2. The methodof claim 1, wherein forming an array comprises forming an array on theback side of the substrate after forming circuitry on the front side ofthe substrate.
 3. The method of claim 1, wherein treating comprisesdoping the subset of the photodetectors with at least one of sulfur,selenium, and tellurium.
 4. The method of claim 3, wherein dopingcomprises forming a mask over the photodetectors that are not of thesubset, applying a chemical vapor deposition of chalcogens over thephotodetectors that are not masked and applying a thermal anneal to thephotodetectors that are not masked.
 5. The method of claim 1, whereintreating comprises irradiating the subset of the photodetectors.
 6. Themethod of claim 5, wherein irradiating comprises irradiating in thepresence of chalcogens to drive chalcogen impurities into the subset ofphotodetectors.
 7. The method of claim 1, wherein treating comprisescausing a silicon surface of the subset of photodetectors to transitionto liquid in the presence of chalcogens and then to rapidly solidifywith some chalcogen incorporated.
 8. The method of claim 1, furthercomprising applying a red, a green, or a blue color filter in arepeating pattern over each photodetector that is not treated.
 9. Themethod of claim 1, further comprising: applying infrared light colorfilters over the treated photodetectors; and applying visible lightcolor filters over the photodetectors that are not treated.
 10. Themethod of claim 1, wherein treating comprises adding impurities to thesubset of the photodetectors without adding impurities to the otherphotodetectors.
 11. The method of claim 10, wherein the impurities aretrapped in a silicon matrix of the subset of the photodetectors.
 12. Themethod of claim 1, wherein treating comprises reducing the effectivesilicon band gap of the subset of the photodetectors.
 13. A methodcomprising: forming an array of photodetectors on a silicon substrate;forming a mask over photodetectors that are not of a subset of thephotodetectors; applying a chemical vapor deposition of chalcogens overthe subset of the photodetectors that are not masked; applying a thermalanneal to the photodetectors that are not masked to improve sensitivityof the photodetector to infrared light; removing the mask; and finishingthe photodetector array to form an image sensor.
 14. The method of claim13, further comprising irradiating the subset of the photodetectorsduring the chemical vapor deposition to drive chalcogen impurities intothe subset of the photodetectors.
 15. The method of claim 13, furthercomprising: applying infrared light color filters over the subset of thephotodetectors; and applying visible light color filters over thephotodetectors that are not of the subset.
 16. A method comprisingforming an array of photodetectors on a silicon substrate; addingimpurities to material of only a subset of the photodetectors to improvesensitivity to infrared light without improving sensitivity to visiblelight; and finishing the photodetector array to form an image sensor.17. The method of claim 16, wherein adding the impurities comprisestrapping the impurities in the silicon matrix of photodetectors of thefirst array of photodetectors.
 18. The method of claim 17, whereintrapping the impurities comprises causing a silicon surface of thesubset of photodetectors to transition to liquid in the presence ofchalcogens and then to rapidly solidify with some chalcogenincorporated.
 19. The method of claim 16, further comprising masking theother photodetectors before adding the impurities and removing the maskafter adding the impurities.
 20. The method of claim 16, wherein addingthe impurities comprises reducing an effective silicon band gap of thesubset of the photodetectors as compared to the other photodetectors.